Currently, light sources such as light emitting diodes (LEDs) or cold cathode fluorescent lamps (CCFLs) are widely used in the lighting industry, e.g., for backlighting liquid crystal displays (LCDs), street lighting, and home appliances. A light driving circuit can be used to adjust power delivered to the light source according to a dimming signal, e.g., a pulse width modulation (PWM) signal.
FIG. 1 shows a block diagram of a conventional light driving circuit 100. The light driving circuit 100 includes an alternating current (AC) to direct current (DC) converter 104, a power converter 106, and a dimming module 112. The AC to DC converter 104 converts an input AC voltage provided by an AC power source 102 to a first DC voltage. The power converter 106 transforms the first DC voltage to a second DC voltage having a voltage level suitable for powering an LED string 108. The dimming module 112 can operate in a burst-dimming control mode, in which the dimming module 112 generates a pulse width modulation (PWM) signal 120 to adjust the power delivered to the LED string 108 so as to regulate the brightness of the LED string 108. More specifically, the light driving circuit 100 further includes a switch 110 coupled to the LED string 108 and operable for controlling a current ILIGHT flowing through the LED string 108 according to the PWM signal 120, which further determines the brightness of the LED string 108.
FIG. 2 shows a timing diagram 200 of signals generated by the light driving circuit 100. FIG. 2 is described in combination with FIG. 1. In the example of FIG. 2, the timing diagram 200 shows the PWM signal 120 and the current ILIGHT flowing through the LED string 108. When the PWM signal 120 is high, e.g., during a time duration TON from t1 to t2, the switch 110 is turned on. The current ILIGHT having a predetermined level I1 flows through the LED string 108. When the PWM signal 120 is low, e.g., during a time duration TOFF from t2 to t3, the switch 110 is turned off. The current ILIGHT drops to substantially zero ampere. Thus, by adjusting the duty cycle of the PWM signal 120, an average level of the current ILIGHT is varied to regulate the brightness of the LED string 108.
However, due to the characteristics of semiconductor devices such as the power converter 106, the current ILIGHT needs a delay time TDELAY to reach the predetermined level I1 after the switch 110 is turned on, e.g., at t1 or t3. As such, the dimming control of the LED string 108 may be affected by frequency noise of the light driving circuit 100. For example, if the frequency of the PWM signal 120 is greater than a predetermined threshold FMAX when the duty cycle is relatively low (e.g., the duty cycle is in a range of 0˜5%), the time duration TON is close to or less than the delay time TDELAY. Thus, the average level of the current ILIGHT does not vary in accordance with the duty cycle of the PWM signal 120, which results in a failure in dimming control of the light driving circuit 100.